Depends on your area and the effciency of the PSU... You could easily save $20-$30 a year if your running a single machine 24/7 and start running it only 12.... Or much less.... And the costs add up with more machines

I run my home server all day every day but when it's not being used, it goes into low power mode... which is kinda like hibernate i guess.... but not quite.

the rest of my computers, I leave them on during the day and turn them off when I go to bed.... but, if you have a high efficiency PSU, and your computer is not being used... it really doesn't use that much power